Generally, epoxy resin cured products produced from an acid anhydride and an epoxy resin are low-cost, and yet exhibit excellent levels of transparency, electrical insulation, chemical resistance, moisture resistance and adhesiveness, and consequently they are used in a variety of applications, including as electrical insulation materials, semiconductor device materials, adhesive materials, and coating materials. An example of a typically representative application is the encapsulant used for protecting the light-emitting element of a light-emitting diode (hereafter, abbreviated as LED). However in recent years, the increasing use of white LEDs, which combine a light source that emits light of shorter wavelength with a fluorescent substance, has meant that deterioration of the encapsulant has become a significant problem.
In other words, because a white LED uses a higher energy light source, the encapsulant is more prone to deterioration and coloring, causing a shortening of the life of the LED, than conventional red and green LEDs. Furthermore, as improvements in the light-emitting element have lead to ongoing reductions in the size and increases in the current carrying capability of the element, the quantity of heat generated by the LED when lit for extended periods has increased, and this heat also causes deterioration of the encapsulant.
The need to suppress this type of deterioration caused by light and heat is a major barrier to more widespread use of epoxy resins. Epoxy resin compositions in which the aromatic epoxy resin, which is prone to deterioration caused by light or heat, is replaced with an alicyclic epoxy resin have been proposed as potential solutions to this problem of deterioration (for example, see Japanese Laid-Open Publication No. 2000-196151, Japanese Laid-Open Publication No. 2003-12896, Japanese Laid-Open Publication No. 2003-26763, and Japanese Laid-Open Publication No. 2003-221490).
However, epoxy resin compositions that use these alicyclic epoxy resins suffer a different disadvantage in that they produce a cured product that exhibits inferior toughness, and is prone to developing cracks under changes in conditions such as the temperature. A method of resolving this disadvantage, which uses a modifying agent comprising any of a variety of polymers to toughen the cured product obtained from the epoxy resin composition, is already known. For example, a method has been proposed wherein by adding a polyester resin to the epoxy resin composition, the toughness of the cured product is improved with no loss of transparency (for example, see Japanese Laid-Open Publication No. 2004-131553).